Heat sensitive printing plate precursors

Abstract

A lithographic printing plate precursor comprises a grained and anodized aluminum substrate coated with a metallic layer, preferably a silver layer, on top of which is applied a layer comprising at least one oleophilising agent and at least one hydrophilic stain-reducing agent, the hydrophilic agent being chosen such that it adsorbs onto the metallic layer but is not so strongly adsorbed thereon as to displace the oleophilising agent. Preferably, the oleophilising agent comprises a mercaptotetrazole or mercaptooxadiazole derivative, the hydrophilic stain-reducing agent comprises a material which includes at least one sulfur, selenium or tellurium containing group, and the layer additionally comprises an additional hydrophilic material. The invention provides lithographic printing plate precursors which may be image-wise exposed by means of a high intensity laser beam to provide press ready plates showing reduced stain in non-image areas, high image quality, good press properties and high durability on press without the requirement for the use of intermediate film and developer chemistry.

1 . A lithographic printing plate precursor comprising: 
 (a) a grained and anodized aluminum substrate, having provided thereon    (b) a metallic layer, on top of which is applied    (c) a layer comprising 
 (i) at least one oleophilising agent which is adsorbed onto the metallic layer; and  
 (ii) at least one hydrophilic stain-reducing agent.  
 said hydrophilic agent being adsorbable onto the metallic layer without displacing the adsorbed oleophilising agent.  
   
     
     
         2 . A lithographic printing plate precursor as claimed in  claim 1  wherein said metallic layer comprises a silver layer.  
     
     
         3 . A lithographic printing plate precursor as claimed in  claim 1  Wherein S lid metallic layer has a thickness of from 1 nm to 100 nm.  
     
     
         4 . A lithographic printing plate precursor as claimed in  claim 1  wherein said oleophilising agent comprises a mercapto, mercaptan or thio derivative.  
     
     
         5 . A lithographic printing plate precursor as claimed in  claim 1  wherein said oleophilising agent comprises a mercaptotetrazole or mercaptooxadiazole derivative.  
     
     
         6 . A lithographic printing plate precursor as claimed in  claim 1  wherein said oleophilising agent is applied at a dry coating weight of between 0.1 and 10 mg/m 2 .  
     
     
         7 . A lithographic printing plate precursor as claimed in  claim 1  wherein said hydrophilic stain-reducing agent comprises a material which includes at least one sulfur, selenium or tellurium containing group.  
     
     
         8 . A lithographic printing plate precursor as claimed in  claim 7  wherein said sulfur, selenium or tellurium containing group comprises a thiol, substituted thio, disulfide, thioacid, thioamide or thiocyanate group, or a selenium and tellurium analogue thereof.  
     
     
         9 . A lithographic printing plate precursor as defined in any preceding claim wherein said hydrophilic stain-reducing agent is present in said layer to the extent of between 10 and 90%.  
     
     
         10 . A lithographic printing plate precursor as claimed in any preceding claim wherein said layer containing at least one oleophilising agent and at least one hydrophilic stain-reducing agent additionally comprises at least one additional hydrophilic material.  
     
     
         11 . A lithographic printing plate precursor as claimed in  claim 10  wherein said additional hydrophilic material comprises a compound capable of desensitizing the non-image areas of the exposed plate precursor to ink.  
     
     
         12 . A lithographic printing plate precursor as claimed in  claim 11  wherein said desensitizer comprises sodium hexametaphosphate, sodium gluconate, dextrin, gum arabic or sorbitol.  
     
     
         13 . A lithographic printing plate precursor as claimed in any preceding claim wherein said layer containing at least one oleophilising agent and at least one hydrophilic stain-reducing agent has a dry coating weight of between 0.01 and 10 g/m 2 .  
     
     
         14 . A method for reducing the background stain of a heat mode laser exposed lithographic printing plate precursor, the said method comprising: 
 (a) providing a lithographic printing plate precursor comprising a grained and anodized aluminum substrate having provided thereon a metallic layer; and    (b) applying, on the metallic layer, a layer comprising 
 (i) at least one oleophilising agent which is adsorbed onto the metallic layer; and  
 (ii) at least one hydrophilic stain-reducing agent.  
 said hydrophilic agent being adsorbable onto the metallic layer without displacing the adsorbed oleophilising agent.  
   
     
     
         15 . A method of preparing a lithographic printing plate, said method comprising: 
 (a) providing a lithographic printing plate precursor as claimed in any of  claims 1  to  13 ; and    (b) image-wise exposing said precursor by means of a heat mode laser beam.    
     
     
         16 . A method as claimed in  claim 15  which additionally includes the step of subjecting the plate to a manual or automatic scrubbing and/or soaking treatment with an aqueous solution.